Java中list.foreach不能使用字符串拼接的问题

final StringBuilder str = new StringBuilder("已选择:");

    @Test
    public void forEachTest(){
        String str = "中国你好！";
        List<String> list = Arrays.asList("a","b","c","d");        
        list.forEach(item->{
         //编辑错误：Variable used in lambda expression should be final or effectively final
            str += item;
        });
        //可以使用增强for
//        for (String item : list) {
//            str += item;
//        }
        System.out.println("结果：" + str);
    }

    @Test
    public void forEachTest(){
        //String str = "中国你好！";
        List<String> list = Arrays.asList("a","b","c","d");
        StringBuffer stringbuffer = new StringBuffer("中国你好！");
        list.forEach(item->{
            stringbuffer .append(item);
        });
        //可以使用增强for
//        for (String item : list) {
//            str += item;
//        }
        System.out.println("结果：" + sb);
    }

    @Test
    public void finalTest(){
        String str = "中国你好！";        
        //在Lambda中使用该变量，该变量不能被修改过，java8会默认加上final
        //str = "c";
        List<String> list = Arrays.asList("a","b","c","d");
        List<String> collect = list.stream().filter(item -> {
            return item.equals(str);
        }).collect(Collectors.toList());
        System.out.println("结果：" + collect);
  //不能改变str，否则Lambda表达式中编译失败
  //str = "山东你好";
    }